Reflaga+Thundraga combo = instant annihilation!
Aside from spell caster Heartless/Nobodies and bosses I could clear out almost all enemies on the field with this combination. I think I could go through almost the whole game just using this combo and not use the Keyblade.
I used thunder the most, as it's THE most accurate. Then fire, as I got surrounded (on purpose, durr) a lot. Then reflect, as it was the best defensive spell (the next being magnet). I didn't use cure all too much, since it completely depleted my MP, and that was basically my four shortcuts.
BLIZZARD WAS BASICALLY POINTLESS. I was quite disappoint in KH2's blizzard attack.
Oh, and higher reflect spells were amazing at clearing the field of dragoons and berserkers.